you mean by righlt clicking and selecting save as? if so, there is some java script you can use, but that is only useful if they right click, i believe.. not sure if there is something specific that will prevent the images from being copied from your site..
People will allways be able to copy images from a website. Most of the methods that try to prevent you (which don't stop you at all) are pretty pointless and anoying IMO. Depending on what the image is watermarking is probably the best option, although in many cases can't be used.